This application is based on, and claims priority to, Japanese application number 09-050738, filed on Mar. 5, 1997, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to the transmission of a wavelength division multiplexed (WDM) signal through an optical fiber transmission line to reduce or eliminate the effects of stimulated Brillouin scattering (SBS).
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. In a typical optical communication system, a light source produces light which is then modulated with data. The modulated light is input to, and transmitted through, an optical fiber transmission line. A receiver receives the modulated light from the optical fiber transmission line, and demodulates the light to obtain the data.
Unfortunately, the transmission quality of the transmitted light will be reduced by stimulated Brillouin scattering (SBS) if the intensity of the light input to the optical fiber transmission line is greater than a predetermined SBS threshold level.
More specifically, SBS refers to a phenomenon in which a nonlinear effect occurs in an optical fiber transmission line when a coherent light from a light source is input to the optical fiber transmission line at a higher intensity than a predetermined SBS threshold level, and the optical incident power is progressively increased. As a result of SBS, a light having higher light intensity is returned to the light source. This returned light has an undesirable effect on the light source.
In particular, SBS has a severely negative influence when the light source is a semiconductor laser and an optical fiber amplifier is used to amplify the light produced by the semiconductor laser.
Therefore, the intensity of light produced by a light source and input to an optical fiber transmission line is limited by the SBS threshold level of the optical fiber transmission line, thereby limiting the transmission distance.
Various conventional techniques are available for suppressing SBS. For example, in a conventional technique, the intensity of light produced by a light source is below the SBS threshold level. See, for example, U.S. Pat. No. 4,560,246.
In an additional conventional technique, the optical frequency or phase of the optical signal input to an optical transmission line is modulated to spread the coherent optical spectrum, to thereby suppress SBS. See, for example, xe2x80x9cNonlinear fiber opticsxe2x80x9d by Govind P. Agrawal, published by the Academic Press, Inc., pages 268 and 269.
SBS also occurs in optical communication systems employing wavelength division multiplexing (WDM). Generally, WDM is used in optical communication systems to transfer a relatively large amount of data at a high speed. With WDM, a plurality of light signals, each modulated with information, is combined into a wavelength division multiplexed (WDM) light. The WDM light is then transmitted through a single optical fiber (an optical fiber transmission line) to a receiver. The receiver splits the WDM light into the individual light signals, so that the individual light signals can be detected. In this manner, a communication system can transfer a relatively large amount of data over a single optical fiber.
In optical communication systems employing WDM, the above-described conventional techniques can be used to suppress SBS. For example, the light signals in the WDM signal can be set at an intensity which is lower than the SBS threshold level of the optical fiber. Alternatively, in optical communication systems employing WDM, the frequency or phase of light signals produced by a light source can be modulated to suppress SBS.
Unfortunately, with the conventional techniques for suppressing SBS, the intensity of a light signal produced by a light source is limited since it must be below the SBS threshold level. As a result, when the number of light signals multiplexed together into the WDM signal is increased, the intensity per light signal will likely be further reduced, thereby reducing the transmission distance of the WDM signal.
It is an object of the present invention to provide a method an apparatus for suppressing SBS in an optical communication system employing WDM, where the intensity of lights produced by light sources can be above the SBS threshold level.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing and apparatus and method for transmitting a wavelength division multiplexed (WDM) signal through an optical fiber transmission line. The WDM signal includes a plurality of light signals multiplexed together. The WDM signal is transmitted to the optical transmission line so that the intensity per light signal is higher than an intensity threshold at which stimulated Brillouin scattering (SBS) occurs for a single light signal individually transmitted to the optical transmission line. Preferably, the WDM signal includes at least six light signals multiplexed together Moreover, the intensity of each light signal can be higher than the intensity threshold at which SBS occurs for a single light signal individually transmitted to the optical transmission line.
Objects of the present invention are also achieved by providing an optical communication system which suppresses SBS. At least six light sources each produce a respective, corresponding light signal. The light signal produced by each respective light source is at a different wavelength than the light signals produced by the other light sources. At least six modulators correspond, respectively, to the at least six light sources. Each modulator modulates the light signal produced by the corresponding light source. A multiplexer multiplexes the modulated light signals into a wavelength division multiplexed (WDM) signal. The WDM signal is transmitted through the optical transmission line so that the intensity per light signal is higher than an intensity threshold at which stimulated Brillouin scattering (SBS) occurs for a single light signal individually transmitted to the optical transmission line.